EFFECTS OF STRAIN AND GRAIN-SIZE ON CARBIDE PRECIPITATION AND CORROSION SENSITIZATION BEHAVIOR IN 304-STAINLESS-STEEL

The simultaneous effects of strain and grain size on carbide precipita tion and sensitization development in 304 stainless steel having a con stant carbon composition of 0.051% were studied. Three different grain sizes (150 mu m, 40 mu m, and 15 mu m) were uniaxially deformed to 10 % and 20% strain, and aged in the sensitization ranges of 625 degrees C and 775 degrees C. 150 mu m samples were also strained to 20% in liq uid nitrogen and aged between 0.1 and 10 h at 670 degrees C. At 625 de grees C, straining of the 150 mu m grain size material produced a syst ematic increase in carbide precipitation, sensitization development, a nd desensitization. Transmission electron microscopy (TEM) revealed a greater continuity of carbides along the grain boundaries in the strai ned samples vs the unstrained samples. At the 15 mu m grain size, thes e strain effects were found to be far less prominent with the sensitiz ation-desensitization process occurring much faster, and accelerated a t the higher aging temperature of 775 degrees C, i.e. sensitization-de sensitization envelopes were shifted to shorter aging times. Liquid ni trogen straining produced a sub-micron, 2-phase gamma/alpha', sub-micr on grain structure which demonstrated an almost instantaneous sensitiz ation-desensitization behavior, when aged at 670 degrees C, which was predicted by extrapolation of the aging data for the larger range of g rain sizes (similar to 15 to 150 mu m) and the 625 and 775 degrees C a ging temperatures. (C) 1997 Acta Metallurgica Inc.